If this consistently has happened at every place you've worked in your career, I think you're luckier than you realize. Independent of the implementation, it's also still interesting as a sign of what might be supported for users down the line; I don't agree that this is how they'd write the code if they never expected to allow another provider, and other comments in the thread have already provided some insight into what the context might be (e.g. EU market).

Apple had enough experience relying on and being screwed by third parties that they have a built-in allergy to it. They do not like being boxed in and always want to be in control of their destiny.

This would’ve been useful during their bake off where they were trying to choose if they wanted to go with Google, Anthropic, or OpenAI. But even without that, they would absolutely not want to be in a situation where they had to redo everything to get off a bad partner.